The present invention relates to a method for bonding a contact made of a silver-based material onto a support at least the surface of which is made of a copper-based material.
Because of their good contact-making properties, silver-based contacts made of Ag, Ag-Cd, Ag-CdO and Ag-Ni are used in contacts for many electrical switching applications. But since silver is expensive, such a contact is usually fixed to a support which is in most cases made of copper or an alloy of copper having a high electrical conductivity. As described, for example, in "Denki Zairyo Manual (Manual of Electrical Materials)" published by Shin-Gijutsu Kaihatsu Center, K.K., pp. 635-638, October 1978, contacts are fixed by caulking, welding or soldering, as is conventionally done to bond metal parts. Caulking is suitable for high-speed automatic operation but it has the following defects: a great caulking force is necessary for fixing a large contact, waste of the contact material is unavoidable, and a hardened silver-based contact softens with time and accordingly the adhesion to the support becomes weaker. Welding which uses heat generated by application of electricity to the contact and support is not often used for fixing silver-based contacts. Silver soldering is the most common and is performed by melting a silver foil between the support and the contact by a gas burner, resistance heating or high-frequency induction heating. But even this technique has problems: expensive silver solder is necessary, insertion of a silver solder foil at a predetermined position is not easily done by automation and requires skilled personnel, the flux used can present an environmental hazard, and gas generated by the flux may shift the position of the contact being soldered or form bubbles in the bonding area. Further, since Ag-CdO alloy does not provide sufficient silver soldering property, one of the surface must be unchanged as Ag-Cd alloy by one-sided oxidation method. Alternatively, Ag layer must be formed onto the one of the surfaces. Therefore, such techniques require troublesome bonding process.
Diffusion bonding can also be used. However, this technique cannot be simply applied on a commercial scale because the side of the contact to be bonded to the support must be made of silver only (as noted in Japanese Published Patent Application (OPI) No. 78060/73) and because a vacuum chamber is necessary for performing the technique (as described in Japanese Published Patent Application No. 61456/76).